The invention concerns a paper machine felt for use in particular in the press section of a paper machine.
The conventional paper machine felt generally comprises a support in the form of a woven fabric, knitted fabric, or thread layer, and a nonwoven fabric needle-felted thereonto on one or two sides. The paper-side nonwoven layer can be of two-stage configuration, in that the nonwoven layer adjacent to the support has coarser fibers than the nonwoven layer forming the paper side.
Materials suitable for the fibers from which the support is manufactured include various plastics, predominantly polyamides (cf. U.S. Pat. No. 4,564,985) and polyesters. In the former case, polyamide-6 and polyamide-6,6 grades have mostly been utilized, in particularly also for the fibers of the nonwoven fabric. In EP-B-0 287 297 it has been proposed to use fibers made of polyamide-12 for the nonwoven fabric, since these fibers have greater resistance to compaction due to their lower water uptake, and moreover are more abrasion-resistant than the polyamide-6 types. Polyamide-11 has similar properties (cf. EP-B-0 372 769).
One problem with the known paper machine felts is that their permeability deteriorates during operation because of compaction of the felt and deposits, even if polyamide-11 or polyamide-12 grades are used. This is counteracted by designing the nonwoven fabric to be very open. This in turn, however, has the disadvantage that the paper machine felt is too open in the initial phase; this promotes, in particular, a tendency for the paper web to become rewetted at the end of the press section. To solve this problem, it has been proposed to mix into the nonwoven fabric soluble fibers that, after a certain operating period, can be dissolved out using a suitable liquid so that the original permeability can thereby be restored (cf. EP-A-0 303 798, DE-A-34 19 708). Such fibers are expensive, however, as is the use of the liquid solvent.
It is the object of the invention to configure paper machine felts in such a way that they have a more consistent permeability over their service life, with no need for additional actions during operation for that purpose.
This object is achieved, according to the present invention, in that there is present in the nonwoven layer or in at least one of the nonwoven layers a fiber combination in which a portion of the plastic fibers is made of a material A that has a water uptake of more than 10 wt % (saturation value, 23xc2x0 C.) and in which the majority of the plastic fibers is made of a material B that has a water uptake of no more than 10 wt % (saturation value, 23xc2x0 C.). The basic idea of the invention is thus to provide in at least one nonwoven layer a fiber combination having plastic fibers with different water uptake properties, the majority of the plastic fibers being constituted by those having the lower water uptake. In this context, the difference between the two materials A and B in terms of water uptake capability should be distinct.
The effect of the fiber combination is that the fibers made of material A, because of their high water uptake, swell up substantially more than those made of material B. Because of this swelling, the paper machine felt acquires the desired permeability only in the paper machine. Because material A is not as strong as the other plastic fibers of the nonwoven layer, however, and in particular not as strong as material B, the fibers made of material A gradually decompose, due to internal friction as well as chemical influences, as the service life proceeds, counteracting the age-related densification of the paper machine felt resulting from deposits and/or compaction. By correspondingly adjusting the proportion of plastic fibers made of material Axe2x80x94the proportion by weight should preferably be between 3 and 25%xe2x80x94and the size of the fibers, it is possible to adjust the permeability behavior for minimal change over the operating time.
In an embodiment of the invention, provision is made for the fiber combination to be present in a paper-side nonwoven layer. As a result, after the plastic fibers have swollen a kind of barrier layer forms, which reduces rewetting of the paper web after leaving the press nip. The reason for this is that the water can pass through the barrier layer under pressure in the press nip, but after leaving the press nip is prevented by the barrier layer, in the absence of any pressure action, from flowing back toward the paper side. The fiber combination of materials A and B can, however, also be present in a nonwoven layer arranged in the interior of the paper machine felt, which is arranged in particular above and/or below a support. As an alternative thereto, provision is made for all the nonwoven layers to have the fiber combination according to the present invention.
It has proven to be advantageous that the nonwoven layer(s) containing the fiber combination has or have a weight per unit area of 50 to 150 g/m2.
The basic concept of the invention can also be carried out, in a paper machine felt having a textile thread system, in that threads made of a material A that has a maximum water uptake of more than 10 wt % (saturation value, 23xc2x0 C.), and threads made of a material B that has a water uptake of no more than 10 wt %, are provided in the thread system. The effect is substantially the same as in the case of the fiber combination described above. The threads made of the polyamide material can be part of a woven fabric, a knitted fabric, or a thread layer that is advantageously additionally arranged in the paper-side nonwoven layer. There also exists the possibility, however, that the woven fabric, knitted fabric, or thread layer having the threads made of materials A and B forms the support of the paper machine felt.
Polyamide-4 and polyamide-4,6 are suitable as material A, since this material is characterized by high water uptake (cf Ulhmanns Encyklopxc3xa4die der technischen Chemie [Ullmann""s Encyclopedia of Chemical Engineering], 4th ed., 1976, Vol. 11, page 296; Rxc3x6mpp-Chemie-Lexikon [Rxc3x6mpp Chemical Dictionary], 8th ed., 1992, Vol. 5, page 3510). Particularly high water uptake values are attained, however, with polyamide-6 and polyamide-6,6 if these polyamides have poly (vinylpyrrolidone) (PVP) and/or polyetherarnide (PEA) added to them, preferably at a proportion of 3 to 18 wt %. This material combination, like polyamides and polyamide-4,6 themselves, has hitherto been recommended only for utilization in the apparel sector (EP-A-0 802 268). Depending on the proportion of PVP, a maximum water uptake of almost 20% can be attained (standard environment: 20xc2x0 C., 65% relative humidity).
Polyamide materials such as polyamide-6, -6,6, -6,10, -6,12, -11, and -12, but also poly (trimethylene terephthalate) or poly (butylene terephthalate), are suitable as material B. The last-named materials, in particular, are characterized by low water uptake, so that when they are used, a definite distinction from material A can be created in terms of water uptake capability.